The present invention generally relates to pressure and temperature transducers and, in particular, to capacitance-type transducer systems for remotely sensing and transmitting variable environmental conditions.
Pressure and temperature transducer systems are well known but, for the most part, the systems have not employed variable capacitors as the operative element. In particular, as has been noted, the present invention primarily is concerned with deriving information from remote or inaccessible positions and, also, with the use of a capacitance-type sensor for this purpose. For example, the present system is particularly concerned with monitoring pressure and temperature of a solid propellant charge of a rocket motor over relatively long periods of time. To accomplish this purpose, transducer capsules are embedded or implanted within the high modulus propellant material. Such a situation is considered to be a remote or inaccessible since, for certain reasons the capsule must be physically unconnected with external power sources or, in other words, must have no electrical coupling power lines. Direct electrical coupling or wires are avoided because they provide a path for temperature dissipation and for pressure leakage. In addition, they reinforce the propellant grain, augment burning rate and produce added stress perturbation.
The need for direct electrical coupling is avoided by the use of what will be termed a `wireless` pressure and temperature transducer that is powered by telemetry links. As far as is known, such systems have not been successfully developed although the reasons for the absence of such developmental work can only be surmised. One problem, however, is that capacitive transducers entail certain difficulties particularly when there is a need to provide a relatively simple and inexpensive system characterized both by a high output level and a output which varies linearly with the sensed variations. Because of the remote disposition of the sensor, high output clearly is desirable. Linearity is important since non-linear outputs require relatively complex and expensive computer arrangements to derive the desired information.
Generally considered, the present invention involves transmitting a signal frequency to a remotely-disposed transducer which, in preferred form, is contained in a capsule deeply embedded in a solid substance such as the previously-discussed propellent charge. Other applications, of course, are contemplated. The transducer of the system includes a variable capacitor exposed to the environment to vary its capacitance with such changing physical conditions as pressure and temperature variations. An inductor carried by the transducer derives the DC power of the transmitted signal and employs this power both to charge the capacitor and to drive a tank circuit that contains the variable capacitance. The tank circuit oscillation provides a transmittable signal frequency receivable at a monitoring center or control station. Since the tank circuit capacitance is the only variable, the oscillations are modulated by the varying capacitance to provide the desired information.
One object of the invention is to provide a so-called `wireless` transducer system for measuring environmental physical condition changes such as pressure and temperature variations.
Another object is to provide pressure and temperature sensing devices based on capacitance change principles which are linked to telemetry systems for both input power and output data transmittal.
A further object is to provide a telemetry-linked transducer system especially adapted for monitoring pressure and temperature changes within a solid propellant charge of a rocket or the like. Other objects will become apparent in the ensuing description.